FIG. 1 shows an example mesh optical network with a number of interconnected nodes. Nodes in the mesh optical network may be configured to be colorless (can receive and/or transmit signals of multiple wavelengths), directionless (can direct signals in multiple directions), and contentionless (can receive and/or transmit multiple signals with the same wavelength within the same node). As shown, a colorless, directionless, and contentionless (“CDC”) network is often implemented with one or more reconfigurable optical add and drop multiplexer (“ROADM”) nodes. Although not shown, a colorless and directionless (“CD”) network may have similar building blocks as a CDC network, but with different add/drop structures. As such, the terms CD network and CDC network (also CD node and CDC node) may be used interchangeably in the descriptions below. A ROADM node may be configured to optically route optical signals to other nodes in the network (“express transit”) in multiple directions (“express paths” or “edges”), as well as to convert the optical signals into electrical signals for transmission to local routers (“locally terminate” or “add/drop”). As the number of nodes and the number of edges increases, for example with the addition of new infrastructure and new channels of communication, the mesh optical network becomes “meshier.”
Although nodes in a traditional CDC network may be contentionless, contentions may still occur along edges between the nodes. For example as shown in FIG. 1, transponder A of Node 1 may be configured to transmit an optical signal of wavelength α to Node 2 (channel A), while transponder B of Node 1 may also be configured to transmit an optical signal of wavelength α to Node 3 (channel B). As such, optical signals from transponders A and B may need to share a same edge between Node 1 and Node 2 along their respective express paths, resulting in a contention along the shared edge. Although the optical signal for one of the contentious channels, such as channel A, can still be transmitted via a different express path using other edges (e.g., dotted lines through Node 5), that express path may not be the most efficient express path in the network.
FIG. 2 shows an example ROADM node in a traditional CDC network. As shown, the ROADM node has three “degrees,” labeled as “ROADM West,” “ROADM East,” and “ROADM North,” each of which may receive incoming signals from and/or transmit outgoing signals to other nodes of the network. The ROADM node may include a number of optical and/or electrical components, such as transponders, multiplexers, demultiplexers, switches, amplifiers, etc. For example, high performance data center interconnect (“DCI”) transponders may be configured to perform optical to electrical conversions with relatively high spectral efficiency on the optical line. Complex add/drop structures (“MUX+DeMUX”) may allow transmissions within the node to be CDC. The ROADM node may be configured to maximize optical routing (without converting to or from electrical signals), such as shown for routing signals between all degrees of the node, and to minimize optical to electrical conversions, such as shown only for local add/drop. As mentioned above, a node in the CD network may have similar components, but with different add/drop structures as those shown in FIG. 2.
FIG. 3 shows example asymmetrical edges connecting nodes along an express path in a traditional CDC network. The asymmetry may be a result of practical constraints, such as available locations where the infrastructure can be built. As shown, some edges, such as the relatively longer edge between Node 1 and Node 2, may require more amplifiers than other edges, such as the relatively shorter edge between Node 2 and Node 3. Since each amplifier adds noise to the optical signals, overall optical noise along an express path is a sum of optical noise accumulated along all edges within the express path. As such, the accumulated noise lowers the spectral efficiency (bit/s/Hz) of data transmission along the express path. Or in other words, the achievable capacity (bit/s) of an optical signal in a fixed frequency or wavelength along an express path is lowered by the accumulated noise. Further, wavelength contentions along edges as described above may further reduce overall spectral efficiency and achievable capacity for data transmission along the express path between Node 1 and Node 3.